as much as I’m someone for second chances the damage done to your son by a relative stranger (at the time of the abuse) isn’t going to be undone with an apology. You’ll have to make a decision between between your son and the faith you have in your boyfriend. Your son would have twice been let down by you. The first time when the abuse happened, and the second in you going back on your word.
I recommend two things. First, I would set up some individual sessions for yourself with a therapist familiar with instances of abuse and domestic violence. Talk to this person about forgiving your boyfriend — and your son’s concern. If this therapy helps, and it seems as if you want to go forward, I would highly recommend a meeting with a family therapist so that you, your boyfriend, and your children can all meet together in the safe confines of a family therapist office. If you’re going to go forward — family therapy will be necessary.